Thermally actuated switch



Filed May 20, 1953 INVENT OR ATTQRNEYS Patented June 22, 1954 THERMALLY ACTUATED SWITCH Earl R. Kebbon, Chatham, N. J., assignor to Weston Electrical Instrument Corporation, Newark, N. J a corporation of New Jersey Application May 20, 1953, Serial No. 356,125

5 Claims.

I This invention relates to a thermally actuated switch and more particularly to a thermally responsive switch which locks out in actuated position and must be manually reset.

While such switches may be employed either to open or to close an electric circuit when the temperature rises above or falls below a predetermined temperature, the invention will be described as embodied in a normally closed switch which may be set to open by a rise in temperature above a selected value and which will not reclose upon a subsequent drop in temperature below that value.

Objects of the invention are to provide thermally actuated switches having operators of bimetallic coil type and in which the bimetallic coil is not stressed upon a temperature decrease after actuation of the switch. An object is to provide a switch having an angularly adjustable shaft operator, a bimetallic spiral coil having its inner end secured to the shaft and its outer end free to move along a circular scale of temperature graduations, and an index arm manually adjustable along the scale to determine the temperature at which the bimetallic coil will actuate the switch. More specifically, an object. is to provide a switch of the character stated in which the switch is retained in actuated position by. magnetic members which may be separated by manual adjustment of the index arm.

These and other objects and advantages of the invention will be apparent from the following specification when taken with the accompanying drawing, in which:

Figure 1 is a plan view of a thermally actuated switch embodying the invention;

Figure 2 is, essentially, a vertical section through the same; and

Figure 3 is a horizontal section on line 3--3 of Figure 2.

In the drawing, the reference numeral l identifies the housing of a snap action switch having a shaft 2 which is adjustable through a preselected relatively small angle a to adjust the contacts thereof from a normal to an actuated position, the normal contact position being either open or closed according to the desired temperature-responsive functioning of the switch. The specific internal construction of the switch is not an essential feature of the invention but, for convenience of description, it may be assumed that the switch is a type V i-14 manufactured by Micro Switch Corporation having contacts which are closed by rotation of the shaft 2 in a counter clockwise direction, as viewed in Figure 3, and opened on rotation of the shaft 2 in a clockwise direction.

The switch housing I is supported above the bottom wall of a cup-shaped casing 3 by a bushing 4 and is secured to the casing by a stud 5 which extends through the housing I and bushing 4, and a nut 6. The operating shaft 2 projeets downwardly from the housing I and is substantially coaxial with the casing 3. -The inner endof a bimetallic spiral coil 1 is secured to the shaft 2, and its component elements are so arranged that the coil expands with increasing temperature. The lower edge of the coil is closely adjacent the bottom wall of the casing 3 which is preferably of metal for good heat transfer from the surface on which it is mounted to the bimetallic coil. A cover 8 of glass or other transparent material is secured to the casing base ture at which the switch is opened comprises 1 a wire of resilient metal having an inner section M journalled in the bushingv H and secured-to a manually adjustable knob [5 by a set screw it, a central section ll substantially parallel to the scale plate In but including an upwardly bent portion I 8 for frictional engagement with the scale plate, and an outer downwardly bent end is which extends into the path of movement of the pointer 13 at the free end of the bimetallic spiral coil 7 as the coil expands with increasing The downwardly bent end is of temperature. the index arm constitutes a stop for the pointer l3 and thus constrains the inner end of the bimetallic spiral coil 1 to move the switch'shaft 2 to actuate the switch to its alternative or open position.

A radial arm 20 is secured to the switch oper ator shaft 2 and carries a vaneor mass 2| of magnetic material which is attracted to and held by a permanent magnet 22 to lock theswitch in actuated position. A reversal of the 1 temperature of the bimetallic spiral coil 1 therefore is not effective'to reset the switch but the radial arm 20 extends into the path of-movement of the outer end l9 of-the index arm, and the switch may be reset by manual rotation of the index arm in counterclockwise direction, as indicated by the arrows in Figure 3, to engage and separate the radial arm 26 and vane 2| and the permanent magnet 22. An angle strip 23 is clamped between the switch housing i and the bushing 4, and projects downwardly into the path of the radial arm 20 to serve as a stop limiting the counterclockwise rotation of the operating shaft 2 into reset or non-actuated position.

Twisted leads 24 extend from the switch terminals 25 to other terminals, not shown, within an insulating material housing 26 which is secured to the casing 3,. and which may be opened to receive the conductors 21 of the controlled circuit. The twisted leads 24 must of course be so arranged that they do not block the counterclockwise rotation of the index arm to reset the switch.

The controlled circuit may be a power supply circuit in the case of equipment of moderate electrical demand or it may include a power relay when the electrical load exceeds the capacity of the snap switch.

The described switch is thermally responsive to open when the temperature of the bimetallic coil 1 exceeds the preselected temperature for which the end [8 of the index arm is set. A comparatively light torque is sufiicient to actuate the microswitch and this is developed by a small change in the temperature of the bimetallic coil 1. This temperature increment must be taken into account when the scale ID is graduated in values of temperature according to the position assumed by the coil pointer l3, 1. e. the index arm should be down-scale below the desired operating point by the amount of temperature increment, which was about 12 or 13 degrees F. with one embodiment of the invention. While the scale I!) could be graduated in temperature values to which the indicating arm should be set for desired operation, it usually is preferable to graduate the scale for cooperation with the pointer [3 to indicate the temperature of the bimetallic coil 1. In this case, actual temperature values are indicated except during intervals when the movement of pointer 13 is blocked by the end IQ of the index arm.

Since the switch requires a comparatively light torque for operation, the frictional resistance to rotation of the index arm is relatively small and readily developed by inserting the inner end 14 of the index arm so far into the knob l5 that the bent section It! presses upon the under surface of the scale plate 10.

The described switch may be employed as a safety switch for interrupting the operation of, a household furnace in the case of an over-temperature. For such use, the base of the casing 3 is held in contact with a surface of the furnace by any convenient clamping arrangement and the switch is connected in series in the power circuit of the burner. Since an excessive temperature usually arises only when there is some defect in the system, the switch is locked-out by the magnetic elements and the furnace can not be again started until the switch is manually reset.

The switch may also be employed on kettles for batchcooking or chemical processing when the heating is to be stopped when the contents reach a preselected temperature.

While the specifically described switch operates upon an over-temperature of the bimetallic spiral coil 1, it is to be understood that the invention maybe embodied in a switch which operates upon a drop in temperature below a preselected temperature.

I claim:

1. The combination with a snap-action switch having a shaft operable through a selected angular increment to adjust the contacts thereof between open and closed position, of a casing in which said switch is mounted, a cover for said casing, a scale plate carried by said cover and having an arcuate scale of temperature graduations coaxial with the shaft of said switch, a bimetallic spiral coil having its inner end secured to said switch shaft, a pointer secured to the outer end of said bimetallic spiral coil and movable along said scale of temperature graduations, and an index arm pivotally mounted on said casing cover coaxial with said graduated scale and having an outer end projecting into the path of movement of said pointer on variation of the temperature of the bimetallic spiral coil in the sense preselected for actuation of the switch, said index arm being frictionally secured to said cover plate and manually adjustable thereon to set its outer end at a desired temperature graduation to arrest the pointerof said bimetallic spiral coil, thereby to constrain the bimetallic spiral coil to turn the switch shaft to actuate the switch.

2. The combination with a snap-action, switch having a shaft operable through a selected angular increment to adjust the contacts thereof between open and closed position, of a casingiin, which said switch is mounted, a cover for said casing. a scale plate carried by said cover and.

having an arcuate scale of temperature graduations coaxial with the shaft ofsaid switch, a bimetallic spiral coil having its inner endsecured to said switch shaft, a pointer secured to the outer end of said bimetallic spiral coil and movable along said scale of temperature graduations,

and an index arm pivotally mounted on saidcasing cover coaxial with said graduated scale and' having an outer end projecting into the path of movement; of said pointer on variation of the temperature of the bimetallic spiral coil in the sense preselected. for actuation of the switch, said index arm being frictionally secured to said cover plate and manually adjustable thereon to set its outer end at a desired temperature graduation to arrest the pointer of said bimetallicspiral coil,

thereby to constrain the bimetallic spiral coil to turn the switch shaft to actuate the switch, a radial arm secured to said switch. shaft and carrying a vane of magnetic material, and a permanent magnet mounted in said casing in position to be contacted by said vane on actuation of said switch, thereby to lock said switch in actuated" position.

3. The invention as recited in claim 2, wherein said radial arm extends into the path of movement of said outer end of said index arm, whereby said switch may be reset to non-actuated position by manual rotation of said index arm to separate said vane from said permanent magnet.

4. The invention as recited in claim 2, wherein.v

secured to a manually adjustable knob at the exterior of the casing cover, said wire being of generally rectangular form with a central parallel to the scale plate and a down-turned outer end, said central section of the wire including a bend for frictional engagement with the scale. plate.

No references cited. 

